1. Field of the Invention
The present invention relates to a method of assembling a shadow mask and an apparatus for carrying out the same and, more particularly, to a method of welding a mask sheet of a small thickness on the order of 50 .mu.m to a frame, and an apparatus for carrying out the same.
2. Description of the Prior Art
A color picture tube, such as a color CRT, is provided with a shadow mask behind a three-color phosphor film formed over the inner surface of its faceplate, to display color pictures in a satisfactory picture quality.
A color picture tube of the Trinitron.RTM. system is provided with an aperture grille, i.e., a shadow mask, formed by attaching a mask sheet under appropriate tension to a steel frame. The mask sheet is formed by forming vertical slits in a thin soft steel sheet.
Referring to FIGS. 5 and 6, when assembling this aperture grille, a frame 4 having A-members 2 and B-members 3, is set on a frame support 11 placed on a base 20. The frame 4 is located at a specified position by a frame locating device 1 having an A-member locating jig 5 and B-member locating rollers 6. Then the A-members 2 are drawn towards each other with turnbuckles. Then, a thin mask sheet 7 is placed on the A-members 2 so that locating pins 9 of the assembling machine are received respectively in reference holes formed therein. Then, the locating pins 9 are moved slightly outward, the upper and lower edges of the mask sheet 7 are gripped with grippers, and then the frame 4 is raised slightly to tighten the mask sheet 7 so that the mask sheet 7 is pressed in close contact with the frame 4. Subsequently, the mask sheet 7 is welded to the A-members 2 by roll spot welding or the like.
If the mask sheet 7 has a thickness on the order of 150 .mu.m, the mask sheet 7 has a sufficient stiffness to maintain its shape and the central slit portion of the mask sheet 7 will not sag when the mask sheet 7 is placed on the frame 4 located at the specified position. However, if the mask sheet 7 has a thickness on the order of 50 .mu.m, which is one-third the thickness of the ordinary mask sheet, to construct a light weight aperture grille, the slit central portion 7a of the mask sheet 7 sags between the A-members 2 of the frame 4 and, consequently, the mask sheet 7 cannot be properly set on the frame 4 and it is difficult to weld the mask sheet 7 to the A-members 2 of the frame 4 by the ordinary welding process. Such a difficulty due to the sagging of the slit central portion 7a of the mask sheet 7 is significant when the mask sheet 7 is a large one for a color picture tube of 30 in. or greater.